Unlike liquid crystal displays (LCDs), OLEDs are emissive displays and do not require backlighting. They are made of mostly organic materials and thus have the advantage of potentially low manufacturing cost, full color capability, light weight and flat structure. They can be used as flat panel displays in many applications such as computer monitors, personal assistant devices, automobile displays, etc.
A basic OLED device as shown in FIG. 1 consists of a transparent substrate 16 made of glass or plastic, a transparent anode layer 21 such as ITO (indium tin oxide), an organic hole transport layer (HTL) 20, such as N,N′-diphenyl-N,N′-bis(3-methylphenyl)(1,1′-biphenyl)-4,4′-diamine (TPD) or N,N′-bis(I-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), a light emitting organic layer 10, such as tris(8-hydroxyquinolinato)aluminum (Alq3), which is also an electron transport (ETL) layer, a cathode 23 made of metal or metal alloys, such as Al, Mg, Mg:Ag and an optional cover glass 24. When a voltage is applied between the cathode and the anode, electrons and holes are injected respectively from the cathode into the ETL layer and from the anode into the HTL layer. The injected electrons and holes migrate towards each other because of the electrical potential between the anode and the cathode, and recombine inside the ETL layer near the interface between the ETL and HTL layers. As a result, energy is released in the form of electroluminescent light, which exits through the transparent substrate. The color of the emitted light is determined by the energy band gap of the emitting layer. In the case of Alq3, usually green color light centered at 530 nm is emitted. Different colors of emitting light can be obtained by using different organic emitting layers, or by doping the same organic emitting layer with different color dyes. White emitting light can be achieved by doping the emitting layer with red (R), green (G) and blue (B) dyes or by stacking R, G, and B emitting layers on top of each other. Full color displays can be realized in several approaches as described in detail in a paper by Burrows et al. in the IEEE Transactions on Electron Devices, Vol. 44, No. 8, 1997, p. 1188-1203. Among these approaches, using three color pixels, red (R), green (G) and blue (B) to form a full color pixel is rather simple and has been commonly used in other direct view flat panel displays such as LCDs.
Unlike thin film electroluminescent displays (TFELs), OLEDs are current-driven devices. The amount of light emitted is directly linked to how much current is passing through the device. Usually, the higher the current, the more light is emitted and the brighter the displays are. Unfortunately, two major problems with current OLED devices are their short lifetime and poor stability, problems that are both directly linked to the driving current. The performance of an OLED degrades quickly when it is driven at a high current level and so does its lifetime. Thus, if the efficiency of the OLEDs can be enhanced, then the driving current can be reduced, and the lifetime and stability of the OLED devices can be improved as well.
The efficiency of the OLEDs is mainly affected by two factors: the internal quantum efficiency and the external extraction efficiency. The first factor indicates how many electrons or holes can be generated and how many of them can recombine and emit photons in the desired spectrum. This factor is determined by the choice of OLED materials and structures, such as the cathode, anode, electron and hole transport layers, etc. The external extraction efficiency indicates how much generated light exits the OLED structure. This factor is mainly determined by the optical thin film structure of the OLEDs, such as the refractive indices and layer thicknesses of all layers in the OLED structure. It is well known that an OLED structure resembles that of a Fabry-Perot microcavity that has been extensively used in solid-state light emitting diodes (LEDs) and vertical cavity surface emitting lasers (VCSELs). But unlike those in LEDs and VCSELs, the cavity effect in OLEDs is much weaker due to the low reflectance of the top layer structure including the hole transport layer and the anode. Many efforts have been made to improve the extraction efficiency of OLEDs by increasing the cavity effect using mirrors with higher reflectance. Such effort did result in the enhancement of the peak wavelength emission in OLEDs; unfortunately, this enhancement is often accompanied by the reduction of the emission at other wavelengths, the width of the emission wavelength band and the viewing angle. These side effects are not desired in display applications, which usually require viewing angles larger than ±60° and an emission bandwidth covering the whole visible spectrum region (400-700 nm).
Furthermore, the cathodes used in OLEDs are often made of metals or metal alloys having high reflectivity. They strongly reflect ambient light and as a result significantly reduce the contrast of OLED devices when they are viewed under strong ambient light illumination. Many methods have been suggested to improve the contrast of OLEDs devices. One approach is to use expensive circular polarizers that allow at most 37% of the emitted light to pass through. Another approach is to use black layers to reduce the reflectance of the cathodes. In particular, the thin film interference black layer approach, first disclosed in U.S. Pat. No. 5,049,780 by Dobrowolski et al. and successfully applied to thin film electroluminescent devices, has been recently applied to OLEDs in several US patents, for example, U.S. Pat. No. 6,411,019, U.S. Pat. No. 6,551,651, U.S. Pat. No. 6,429,451, and U.S. Pat. No. 6,608,333. If a perfect black layer with zero reflectance was used, then there would be no cavity effects at all, and the amount of light emitted from such an OLED device would be about four times less than that of a similar OLED device having a conventional cathode with a high reflectance. Furthermore, if the black coating is not perfect and has some residual reflectance, which is often the case in real life, the residual reflectance could result in a cavity effect. The emission can be either enhanced or reduced by this cavity effect, depending on the critical factors of the phase changes on reflection of the two mirror structures. The phase changes on reflection are not considered in the above US patents. In addition, such interference black layer coatings described in these patents require the use of at least one transparent (or semi-transparent) layer and one absorbing layer, all inserted between the conventional metal cathode and the emitting layer. In OLEDs these added layers are required to be conductive and the layer next to the organic emitting layer is required to have a low work function (preferably <4 eV) in order to allow electrons to be injected easily into the emitting layer. The number of transparent conducting materials is limited and these often have high work functions (e.g. 4.4-4.8 eV for ITO) and thus are not suitable to be used next to the emitting layer on the cathode side. In addition, the fabrication of transparent, dense and high-conductivity materials usually requires high-temperature deposition, which is not acceptable when depositing on organic materials, most of them being temperature sensitive. Low-temperature deposited transparent materials have been used for the fabrication of interference black layer coated cathodes. In addition, semi-transparent metal/dielectric mixtures have been proposed to replace the transparent layer in the thin film interference black layers for OLED devices. Such replacements result in poorer black layer performance than those used in TFELs because their optical constants are less suitable for the design of lower-reflectance black layer coatings. Moreover, these semi-transparent layers are deposited by the simultaneous evaporation of metal and dielectric materials. The properties of these co-deposited layers, such as their conductivities and optical constants, vary greatly with the deposition rates of the two materials [Han et al. J. Appl. Phys. 96, 709 (2004)], and so the performance of the OLEDs incorporating such black layers will be also be affected. Furthermore, such black layer coatings are not effective in reducing the light reflected from other interfaces of the OLED devices.